A mobile terminal for communication via satellite is typically equipped with a mobile antenna that keeps pointing towards the satellite while the terminal is in motion. A controller coupled with the antenna uses one or more devices, for example a GPS receiver, a compass, 3-D gyroscopes and accelerometers, for determining a direction of movement and corrects the antenna alignment in accordance with the determined direction of movement to keep the antenna pointing towards the satellite. However, compasses often suffer from large errors, whereas pointing data from gyroscopes and accelerometers tend to slowly drift. In order to compensate for the drift in pointing data and provide slow correction of the antenna pointing, the antenna controller often uses information related to the reception quality of a received signal, for example any one of Signal to Noise Ratio (SNR), Signal Strength, Receiver Signal Strength Indication (RSSI) and Bit Error Rate (BER).
When the mobile terminal is operational both to receive and transmit, the transmitting (TX) antenna and the receiving (RX) antenna usually share the same reflector. Pointing the shared aperture (reflector) in accordance with a reception SNR at the mobile terminal is usually sufficient for achieving fairly accurate pointing, both for receiving and for transmitting.
However, sometimes the direction of the TX antenna beam has a slight squint in reference to the direction of the RX antenna beam, even when both the TX antenna and the RX antenna share the same reflector. Sometimes, the RX antenna and the TX antenna have independent apertures. For instance, in phased array antennas, the TX antenna and the RX antenna usually have independent apertures and although the two antennas may be installed in parallel and the RX antenna pointing may be used for pointing the TX antenna as well, small inaccuracies lead to some squint between the pointing of the two antennas.
In satellite communications, the transmission frequencies are higher than the reception frequencies. Consequently, where an RX antenna and a TX antenna share a common reflector the TX antenna has a narrower beam compared to the beam of the RX antenna. In order to use SNR readings of a received signal for mobile antenna tracking, the antenna is dithered around the pointing direction in order to estimate a gradient of the pointing error, which can then be used for determining the needed pointing correction. One dithering method includes moving the antenna in accordance with four corners of a square around the current pointing direction (e.g. lower-left, lower-right, higher-right, and higher-left). Another dithering method includes moving the antenna in accordance with a conical scan around the some pointing direction. Typically, this dithering or conical scan follows a −0.5 dB contour of the RX beam (the specific contour selection is a compromise between the need for low antenna gain loss and good antenna dynamics). As the TX antenna has a narrower beam, this dithering or scan corresponds to a more degraded contour (for example, a −1 dB contour) around the TX antenna pointing, causing greater loss in the terminal's TX link, even with no squint at all. When some squint is added, the loss in the terminal's TX link is even larger.